Remove redundant wheel dep from pyproject.toml

Remove the redundant `wheel` dependency, as it is added by the backend
automatically.  Listing it explicitly in the documentation was
a historical mistake and has been fixed since, see:
f7d30a9529

While Pillow uses a custom backend that modifies the `bdist_wheel`
method, it does not import `wheel` or use it in a way that would rely
on setuptools implementation details.

pyproject.toml

@@ -2,7 +2,6 @@
 build-backend = "backend"
 requires = [
   "setuptools>=67.8",
-  "wheel",
 ]
 backend-path = [
   "_custom_build",